This invention relates to a hydraulic rotary actuator for use in industrial robots and the like.
Generally, the actuator to be incorporated in a joint of an articulated robot for industrial use must be compact in size, light in weight, quick in response and capable of high speed operation with a high output torque, and have a high degree of accuracy and precision in determination of position.
In recent years there have been proposed various types of hydraulic actuators for use in industrial robots. One known actuator comprises a harmonic drive or speed changer in combination with a hydraulic motor. The harmonic drive comprises a rigid circular spline having a cup-shaped casing and internal teeth, and a flexible spline having a thin-walled cup-shaped casing of flexible material and external teeth, the casing of the flexible spline being disposed within the casing of the rigid circular spline to define a chamber, in which a wave generator is arranged so as to act on the flexible spline to cause selective and partial engagement of the external and internal teeth of the two splines. The hydraulic motor is disposed in the chamber of the harmonic drive, with the wave generator being mounted on the rotatable casing of the hydraulic motor.
Indeed, the actuator of the above-mentioned known construction is not only capable of producing a high output torque since the output speed of the hydraulic motor is reduced by the harmonic drive, but also it can be made compact in size and light in weight since the hydraulic motor is completely enclosed in the casing of the harmonic drive and the wave generator of the harmonic drive is mounted on the casing of the hydraulic motor. However, the above advantages are not sufficient for the conventional actuator to be used effectively in an articulated robot.
To work effectively as a drive for, say, an industrial robot, the actuator must be able to move the robot arm to, or stop it at, a required position against the force of inertia at a high speed and with a high degree of accuracy and precision. When an accident or disorder such as power failure has happened, the robot must be instantly stopped for safety, and when the accident or disorder has been removed, the original position of the robot must be restored as quickly and accurately as possible.
To meet the requirements the actuator must be provided with a detector capable of detecting the position of the robot arm with a high degree of accuracy and precision, and a brake capable of stopping the arm quickly and exactly at a required position against the force of inertia. It has been proposed to provide the actuator with an electromagnetic brake enclosed in the previously mentioned chamber of the harmonic drive so as to act on the hydraulic motor. However, since the chamber is a space defined by various mechanical parts or members of magnetic material, the magnetic flux produced by the coil of the electromagnetic brake tends to be dispersed, with resulting decrease in the effective magnetic force required for proper operation of the brake. To provide a sufficient magnetic force, the brake must be provided with a larger electromagnetic coil, which would occupy a larger space with resulting increase in the size of the actuator.
The magnetic field produced by the coil of the electromagnetic brake magnetizes the various members about the coil, particularly, the supporting shaft of the hydraulic motor. When magnetized, the shaft tends to attract particles or dust of iron or the like contained in the working fluid for the hydraulic motor, so that the particles or dust cause damage to the shaft or the associated members.
For accurate and precise positioning of the robot arm, it has been proposed to provide the actuator with a first and a second rotation detector at the input and output sides, respectively. In the prior art arrangement, however, the two detectors are provided outside the casing of the actuator, so that it is difficult to make the actuator compact, and a feedback gearing must be provided for transmission of the angular position of the output shaft to the detectors, so that the backlash of the gears tends to deteriorate the accuracy and precision of the detecting operation of the detectors.
Accordingly, the primary object of the invention is to provide a hydraulic rotary actuator which comprises a hydraulic motor and a harmonic drive having an interior chamber in which the hydraulic motor is completely enclosed in such a particular relation to the harmonic drive as to eliminate the disadvantages of the conventional devices.
Another object of the invention is to provide such a hydraulic rotary actuator as aforesaid which is compact in size, light in weight, quick in response and operation, and has a high output torque and a high degree of accuracy and precision in operation.
Another object of the invention is to provide such a hydraulic rotary actuator as aforesaid which is provided with an electromagnetic brake for the hydraulic motor and means for increasing the performance of the brake.
Another object of the invention is to provide such a hydraulic rotary actuator which is provided with a first and a second rotation detector enclosed in the chamber of the harmonic drive and arranged in a particular relation to the hydraulic motor and the harmonic drive so that quick, accurate and precise positioning can be effected.
The invention will be described in detail with reference to the accompanying drawing.